1. Field of the Invention
This invention relates generally to the field of power distribution and specifically to a secondary power system.
2. Description of Related Art
Power supplied to the public is generally reliable, but weather and other circumstances can lead to interruptions in the power supply lasting seconds, minutes or hours. In rural areas, these power interruptions are more frequent and of longer duration than in urban areas. An extended power interruption means that power is not available for sump pumps, refrigerators and other vital appliances. Many people have vacation homes in rural areas which are unattended for long periods of time. When power is interrupted and the home is unattended, flooding, food spoilage and other problems can arise. Thus, it is desirable to have available a secondary power supply which does not require the presence of an operator.
Although power is more reliable in urban areas, even a brief interruption can have adverse results. Businesses which rely on computers can be affected by a momentary power interruption. Thus, it is desirable to have a secondary power system which ensures an uninterrupted power supply.
Myriad systems for providing secondary, or emergency, power have been devised. Among them are generators which are started manually or automatically, however, to provide uninterrupted power with a generator based system, the generator must be running at all times and an instantaneous transfer must be possible upon a failure of the normal power supply. Many generators are driven by gasoline engines which must be refueled periodically. This may be impractical in a vacation home which may be vacant for an entire season.
A system which uses an alternating current generator requires switching from one AC supply to another. Connecting together different alternating current power supplies creates problems when the phase and frequency do not precisely correspond. Thus, these emergency power systems require a transfer switch to selectively connect the preferred power supply to a distribution system. Transfer switches can be manual or automatic. Manual transfer switches require the presence of an operator and are slow. Automatic transfer switches are faster, but still do not provide uninterrupted power because of their response time. Faster electronic automatic bus transfer switches have been developed, however, even an instantaneous transfer switch would create safety problems such as the inability to isolate a fault immediately downstream from the transfer switch.
Also common are batteries which are connected to inverters to supply alternating current when the normal, commercial power supply is interrupted. These also require the ability to transfer power and the incident problems. In addition, if the power is to be supplied for any length of time, the batteries must be large and will require significant maintenance.
Systems which generate power simultaneously with the normal power supply have been proposed. These are designed to sell power back to the utility when the power generated exceeds the amount required. This type of system requires the generator to be running regularly and presents certain safety and regulatory obstacles.
Thus, it is desirable to provide an emergency power system which provides uninterrupted power with a minimum of maintenance and a minimum amount of energy consumption.
A related problem is the difficulty in installing a backup power system. Home distribution panels are not designed for connecting additional power supplies to the panel. In many cases, the utility company must shut off power in order to make such a connection. Because of this panel design and the aforementioned problems of connecting an AC supply to the commercial power supply, a means for connecting a backup power system without involving the utility is desired.